This invention relates to aqueous flooding processes, wherein polymers are dispersed in an aqueous liquid and thereby provide mobility control.
Enhanced oil recovery (EOR) by flooding has become widely practiced by the petroleum industry. In conventional enhanced oil recovery processes, an aqueous flooding liquid is injected into the subterranean formation through a pattern of injection wells which surround one or more producing wells. The flooding liquid acts as an oil-immiscible front which displaces oil from the formation and forces it to the production well. In order to maximize the displacement efficiency of the flooding liquid, it has been a practice to add various materials to the medium to increase its viscosity.
As taught in Encyclopedia of Polymer Science and Technology, Interscience Publishers, Vol. I, 192 (1964), it is known that the viscosity of an aqueous medium is increased by the addition of a water-soluble polymer. Such water-soluble polymers include polyacrylamide, acrylamide/acrylic acid copolymer, sodium polyacrylate, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, polysaccharide as well as naturally occurring gums such as guar gum and chemically modified gums such as hydroxypropyl guar gum.
Unfortunately, however, the aforementioned conventional water-soluble polymers suffer from many serious deficiencies or limitations in actual use in enhanced oil recovery. For example, for reasons of efficiency and economical considerations, it is common to employ very high molecular weight versions of such polymers. However, during the injection stage of the EOR process (i.e., the pumping of the liquid into the formation), the aqueous medium containing the high molecular weight water-soluble polymer is exposed to high shear. Such shear often causes mechanical degradation of the polymer and thus reduces the viscosity of the aqueous medium. While lower molecular weight polymers are less sensitive to shear degradation, they must be used in much higher concentrations in order to achieve the desired level of viscosity.
Secondly, while ionic water-soluble polymers such as neutralized acrylamide/acrylic acid copolymer, sodium polyacrylate, polystyrene sulfonate and the like are more efficient thickeners in deionized water than their nonionic counterparts, their thickening ability is greatly reduced by the presence of electrolytes such as sodium chloride, calcium chloride and magnesium sulfate in the aqueous medium. Such electrolytes are commonly present in the ground waters (brines) of subterranean formations subjected to EOR process.
Finally, in many EOR processes, the aqueous medium thickened with water-soluble polymer is exposed to temperatures in the range of 30.degree. C. to 100.degree. C. which normally causes reduction of viscosity. Such high temperatures are particularly common in EOR processes wherein the aqueous medium is pumped underground to depths of 5,000 to 20,000 feet, as is common for mobility control fluids and packing fluids.
In attempts to overcome some of the aforementioned deficiencies of the conventional water-soluble polymers, it has been a common practice to cross-link the polymer in order to improve resistances to thermal as well as shear degradation. See, for example, U.S. Pat. No. 3,247,171. Such attempts have generally not been successful. More recently, as taught in U.S. Pat. No. 3,984,333, an aqueous medium has been thickened by dissolving a water-soluble block copolymer having water-soluble blocks and water-insoluble blocks in the aqueous medium. While such water-soluble block copolymers apparently exhibit reasonably good resistance to shear degradation, such polymers are difficult and often impractical to prepare. More importantly, such polymers do not exhibit significant tolerance of electrolytes normally present in the aqueous media to be thickened.
While the cellulosic derivatives such as hydroxyethyl cellulose and biopolymers exhibit acceptable tolerance to the presence of electrolytes, cellulosic derivatives are generally ineffective at the low concentrations that are economical and exhibit poor thermal stability. The biopolymers such as xantham gums exhibit acceptable thermal stability, resistance to shear degradation and electrolytic tolerance. Unfortunately, such biopolymers are generally very expensive and are susceptible to biodegradation.
In view of the aforementioned deficiencies of conventional water-soluble polymers as mobility control agents in enhanced oil recovery process, it is highly desirable to provide an inexpensive EOR process which employs an agent which exhibits thermal stability, electrolytic tolerance and good resistance to shear and biological degradation.